Errors and Exception-Handling in Visual Basic .NET

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55 Errors and Exception-Handling
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Sub Main() Dim numerator, denominator, quotient As Integer ConsoleWrite("Enter two numbers separated by ENTER: ") Try numerator = ConsoleReadLine() denominator = ConsoleReadLine() quotient = numerator \ denominator ConsoleWriteLine("{0}/{1}={2}", numerator, _ denominator, quotient) Catch ConsoleWriteLine("Error in input") End Try End Sub Listing 546: Code that could cause several types of problem
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In the code shown several types of input could cause problems The user could enter a number for numerator but a non-numeric value for denominator or vice-versa, or the user could enter 0 for denominator The rst two problems would cause errors due to the attempt to enter non-numeric data into a numeric variable The third would cause a divide-by-zero error, since in computer terms (as in mathematics) this is impossible The way the Catch block is formed in Listing 545, the error would be caught and reported and a crash would be averted However, we could improve the error-handling by telling the user what went wrong, as shown in Listing 547
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Sub Main() Dim numerator, denominator, quotient As Integer ConsoleWrite("Enter two numbers separated by ENTER: ") Try numerator = ConsoleReadLine() denominator = ConsoleReadLine() quotient = numerator \ denominator ConsoleWriteLine("{0}/{1}={2}", numerator, _ denominator, quotient) Catch invalidEx As InvalidCastException ConsoleWriteLine("One entry is not a number") Catch div0Ex As DivideByZeroException ConsoleWriteLine("Attempt to divide by zero") End Try End Sub Listing 547: Explicit handling of speci c exception types
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The code in Listing 546 recognizes that there are two possible sources of input error The rst of these is caused by the user entering a non-numeric value into a numeric variable This is called an invalid cast error, the name coming from the use of a variable to perform a role it is not quali ed for A miscast actor is given a role that is outwith their specialism, like asking WC Fields to be a romantic lead A miscast variable is similarly used to store information it cannot deal with, like passing a string into a numeric variable
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5 n Controlling Program Code
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The second error the code deals with is the divide-by-zero problem In either case, NET provides a purpose-built Exception class for dealing with it You can examine the range of Exception classes provided by NET by looking up Exception Hierarchy in the Visual Studio help system
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Exercise 59
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The activity at the end of 4 was to build a simple calculator in which the user enters an expression and this is evaluated You should now see that there are several situations in which the program will fail or crash, these being: a) b) c) d) non-numeric input from the user; division by zero; the user entering an operator that is not supported; numeric over ow (operation results in a value that exceeds the range of the variable it is assigned to) or under ow (operation results in a value that is too small for the precision that the variable it is assigned to can store)
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For each of the possible error types above, try to enter data into the program to nd out how the program will react (ie whether it will crash or simply provide a wrong answer) Add exception handling code to cope with the error results
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552 Structured Exception-Handling
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The mechanism used by Visual Basic and NET to handle run-time errors is known as Structured Exception-Handling (SEH), since the error-handling code is organized as a program structure similar in format to the other code structures we ve looked at in this chapter The general format for an exception-handling structure in Visual Basic NET is shown in Figure 54 The general outline of exception-handling shown in Figure 54 demonstrates the main features of SEH
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Code is grouped into blocks in SEH, with speci c blocks for code that causes exceptions, code that handles exceptions and clean-up code Code between Try and the rst Catch block will execute completely, or until an exception occurs If an exception occurs, NET will transfer control to the rst block that handles the exception If no block handles the exception that happened, the program will behave just as it would have if no exception-handling was used most usually, it will crash If a less speci c handler for an exception appears in the code before a more speci c one (eg Catch ex As Exception occurs before Catch invalidEx As InvalidCastException) it will catch the error Because of this, you must be careful to list exception handlers in order of their speci cness to errors If Catch ex As Exception occurs rst in the list, none of the others will ever execute
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